1. Field of the Invention
This invention relates to a method and apparatus for effecting condensation heating, and more specifically, for vapor phase or reflow soldering, sometimes known as condensation soldering. This is a process wherein the vapors of a high temperature boiling point liquid are allowed to contact the objects or components to be soldered, such as printed circuit boards or other electronic devices. The latent heat of vaporization given up when the vapors condense on the object quickly raise its temperature and cause any solder, tin, alloy or tin-lead electroplate, or other similar material thereon to melt and flow. It is particularly desirable to accomplish this end with minimal loss of the heat transfer fluid, and maximum utilization of energy conserving potential from the heat transfer stages of the device.
2. Description of the Prior Art
Many processes, such as curing, cooking, fusing and brazing, as well as soldering, require that articles be rapidly heated to elevated temperatures. Although the background of the present invention is most readily understood in the context of soldering, its application is not to be construed as limited to soldering.
Several methods and apparatuses have been disclosed in the prior art for effecting solder reflow operations on printed circuits through the use of hot saturated vapors. One such device is disclosed in R. C. Pfahl, Jr. et al, U.S. Pat. No. 3,866,307, issued Feb. 18, 1975. Circuit boards are loaded on to a conveyor and moved downward into a chamber containing hot saturated vapors of a high boiling point fluid such as a fluorinate hydrocarbon, which is relatively expensive. As the circuit boards pass through the vapors they are heated to a suitable temperature for soldering. The heated boards may also then be passed over a solder wave at a low point of the conveyor catenary, which is positioned to allow the solder to contact the component leads and the conductive material on the boards. The soldered boards continue to travel upward past the vapors and out through a port located at or near the top of the chamber.
To limit the loss of expensive heat transfer fluid through the entry and exit ports, the device provides condensation coils to condense the escaping vapors and return them to the chamber. One problem resulting from this approach is that the conveyor cannot follow a horizontal path but must carry the circuit boards downward into the vapor chamber and then upward to and through the exit port. This down and up motion is industrially inefficient and results in pooling of solder at the trailing edge of the circuit boards due to the slanting of the boards on the conveyor belt. Another problem is that despite the use of condenser coils a substantial quantity of heat transfer fluid which has condensed on the boards and the conveyor is carried out of the chamber.
Another device disclosed in T.Y. Chu et al, U.S. Pat. No. 3,904,102 issued Sept. 9, 1975 attempts to reduce loss of the relatively expensive heat transfer fluid by use of a secondary blanket of a vapor of a less expensive fluid atop the primary vapor zone. While this device reduces the loss of heat transfer fluid, a considerable amount is still carried out with the circuit boards. Further, this device also utilizes the up and down, slanted conveyor path described in the Pfahl patent above.
U.S. Pat. No. 4,032,033, T.Y. Chu et al, discloses a solder reflow apparatus which adds a quenching operation to cool the circuit board after the soldering or reflow has been completed.
Another related application of vapor phase soldering is disclosed in H.H. Ammann, U.S. Pat. No. 4,115,601 issued Sept. 19, 1978. This device heats a pre-tinned, flexible printed circuit board to the point where the solder thereon will fuse with the conductive foil etched on the printed circuit board. The Ammann apparatus is designed so that the vapor heating/solder melting phase is carried out while the flexible printed circuit board is moving in a horizontal plane, thus eliminating the solder pooling effect inherent in the two patents discussed previously. However, the Ammann patent has the limitation that it can only be used to solder bare flexible printed circuit boards without components or surface irregularities, and is not equipped to handle rigid printed circuit boards or boards with components mounted thereon. Furthermore, the circuit board path described by the Ammann patent is only horizontal during the solder heating phase. The overall conveyor path follows a sequence of numerous sharp bends which may make the device prone to mechanical failure, construction difficulties and the aformentioned industrial inefficiency cited in the two previous references.